Sulfur mining with steam

ABSTRACT

This is an improvement and modification of the Frasch-type underground sulfur-mining process. The primary source of heat is steam with temperature up to 600*-800* F. which is injected into the sulfur deposit. Hot water at a temperature above 240* but not over 320* F. is injected below the steam. The steam zone is surrounded by an advancing zone of hot condensate within which melting of the sulfur occurs in a temperature range of 240*-320* F. The molten sulfur flows by gravity towards the central production tubing intake and is insulated from the hot injected steam by a blanket of hot water. Reduced water requirement per ton of sulfur and higher thermal efficiency are effected by this process.

ilnited States Patent Inventors Appl. No.

Filed Patented Assignee Clifton S. Goddln, Jr.;

Karol l... Hujsak, both of Okla.

Dec. 19, 1969 Dec. 28, 1971 Amoco Production Company Tulsa, Okla.

SULFUR MINING WITH STEAM 7 Claims, 2 Drawing Figs.

US. Cl

Int. Cl

166/269 E21!) 43/28 Field of Search 299/4, 5, 6; 166/269 References Cited UNITED STATES PATENTS 461,429 l0/l89l Frasch LIFT AIR 545 F STEAM IOOO PSI 4o HoT WATER 1,846,358 2/1932 Reed 3,421,583 1/1969 Koons Primary Examiner-Ernest R. Purser Attarneys- Paul F. Hawley and John D Gassett ABSTRACT: This is an improvement and modification of the Frasch-type underground sulfur-mining process. The primary source of heat is steam with temperature up to 600-800 F.

which is injected into the sulfur deposit. Hot water at a temperature above 240 but not over 320 F. is injected below the steam. The steam zone is surrounded by an advancing zone of hot condensate within which melting of the sulfur occurs in a temperature range of 240-320 F. The molten sulfur flows by gravity towards the central production tubing intake and is insulated from the hot injected steam by a blanket of hot water. Reduced water requirement per ton of sulfur and higher thermal efficiency are effected by this process.

MOLTEN SULFUR PLUS AIR 250-300F WATER PATENTEU UEC28 r971 SHEET 1 0F 2 LIFT AlR-- 320 WATER-- R w Aw m m H O 2 WV fl 5:: R p E O TF A0 8 W0 2 T3 0 H RG W W Lm U Z B PRlOR ART PATENTED UEEZS I97! SHEET 2 [IF 2 LIFT AlR-- MOLTEN SULFUR v PLUS AIR 545 F STEAM IOOO PSI SULFUR BEARING ZONE 32 lNSULATlON\ INVENTORS CLIFTON S. GODDIN JR.

BY 1 KAROL L. HUJSAK ATTORNEY SULFUR MINING WITH STEAM BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to the mining of underground deposits of sulfur. It relates especially to a system in which steam and hot water are injected to accomplish heating of the formation and recovery of molten sulfur.

2. Setting of the Invention Large quantities of elemental sulfur are found in underground deposits which vary in depth from about 300 to 2,300 feet. The most common method of mining this sulfur is by the Frasch hot water process. In that process a well bore is drilled into the sulfur deposit. A central production tubing is installed within an outer, larger diameter casing. Hot water is injected down the annulus between the production tubing and t the casing and flows out through perforations in the casing into the sulfur deposit. When the deposit temperature reaches 240 F. the sulfur melts and flows downwardly toward the production tubing. Compressed air is introduced into the production tubing to lift the molten sulfur to the surface. The hot water temperature is limited to 320 F. because at higher temperature the viscosity of molten sulfur rapidly rises to as high as 10,000 centipoises.

It is known that it is more efficient to transfer heat by steam than with water. However, steam is not used in this type sulfur mining process since the pressure of 320 F. saturated steam is only 75 p.s.i.g. which is insufficient to accomplish injection into sulfur deposits which lie below about 200 feet in depth. For steam to exist above 75 p.s.i.g., the temperature would be above 320 F. and the sulfur would be too viscous to flow through the production tubing.

This invention discloses a system whereby steam can be used to transfer the bulk of the heat to the sulfur deposit yet still permit the sulfur to flow.

BRIEF DESCRIPTION OF THE INVENTION A borehole is drilled into a sulfur deposit and is lined with a casing. A steam injection tubing is inserted with the lower end in communication with perforations in the casing in the lower part of the sulfur deposit. A second hot water tubing is installed inside the casing with its lower end in communication with the sulfur deposit below the steam injection perforations. Inside the water injection tubing is a sulfur production tubing. Inside the sulfur production tubing is an airlift pipe. Hot water is injected through the hot water tubing at a temperature of from about 280320 F., which maintains the flowing sulfur in a low viscosity range. The steam provides the bulk of the heat injected into the sulfur deposit. There is created an inverted cone-heating zone. The steam zone is surrounded by a zone of hot condensate within which melting and gravity drainage of the molten sulfur occurs at 240-320 F. The molten sulfur flows toward the inlet of the sulfur production string. The hot water flowing from the wall casing provides a blanket which insulates this molten sulfur from the higher temperature steam zone.

DESCRIPTION OF THE DRAWINGS FIG. 1 illustrates the conventional Frasch process;

FIG. 2 illustrates the improved sulfur mining process of our invention.

Attention is first directed to FIG. I for a brief description of the prior art method of sulfur mining and to point out some of the limitations which have been found therein. Shown therein is a casing which has been set in a well bore drilled into the sulfur deposit I2. A sulfur production string 14 is suspended concentrically within casing 10. A small string of pipe 16 is suspended within the production tubing 14 so that lift air can be injected into the molten sulfur within the production tubing. An annular packer 18 is set toward the lower end of the casing to direct hot water out the perforations 20 at the desired elevation. As mentioned above, the temperature of the injected water is limited to 320 F. since the viscosity of liquid sulfur rapidly increases from about 10 centipoises at this temperature to above l0,000 centipoises at 325 F. An attempt to operate the conventional Frasch well with injection fluid temperature above 320 F. would be futile due to flow resistance offered by the high-viscosity sulfur. Although steam at 320" F. has substantially more available heat than water at the same temperature, steam is not used in the Frasch process. This is due to the face that the sulfur deposits occur at depths which require injection pressures higher than the 75 p.s.i.g. saturation pressure of 320 F. steam.

In the process of FIG. I, the hot water melts the sulfur and forms a molten sulfur zone 22. The sulfur gravity drains to the base of production string 14. Air is injected down pipe string 16 to lift the molten sulfur to the surface.

The Frasch process has been very successful and accounts for the bulk of the commercial production of elemental sulfur. However, the thermal efficiency of the average Frasch installation is low, ranging from about 10-20 percent. A part of the heat is lost by thermal conduction to adjoining formations, but the bulk of the loss is by fluid transport, i.e., by escape of hot injection water from the zone of effective sulfur recovery.

Attention is now directed to FIG. 2 which shows the improved sulfur-mining process of our invention. External casing 30 is set in a hole drilled into a sulfur deposit 32. Sulfur production tubing 34 is installed inside a hot water injection pipe 36. Inside the sulfur production pipe 34 a small-diameter pipe 38 is installed for injecting lift air. Insulated steam injection tubing 44 is installed inside casing 30 alongside water injection pipe 36. The lower end of steam injection tubing 44 terminates below packer 46 and above packer 40. Thus, steam injected through casing 44 flows out perforations 42 into the sulfur deposit.

Hot water injection pipe 36 extends downwardly through packer 40 in the bottom of casing 30. This hot water pipe is set a short distance above the bottom of the sulfur deposit. An annular seal or packer 50 is provided between sulfur production tubing 34 and hot water injection pipe 36. Hot water injection perforations 52 are provided above packer 50. Thus, hot water is injected below the steam and above the molten sulfur zone 48. The molten sulfur flows toward the lower end of production pipe 34, and is produced in the usual manner by injecting lift air through pipe 38 at a point within production pipe 34.

in our process the bulk of the heat input to the sulfur deposit is provided by the injection of high-pressure steam. For illustrative purposes, 1,000 p.s.i.a. steam (545 F. saturation temperature) is used, which permits injection at a bottom hole depth of 2,300 feet. The pressure of the steam, of course, is adjusted to correspond to the formation pressure. The steam is conducted down through tubing 44 which is insulated to minimize heat transfer to the waterflow in string 36. The injec tion of the hot water in the annular space surrounding the production string 34 serves to control the temperature of the molten sulfur within the low viscosity range. The hot water is injected at the surface into pipe 36 at a temperature above 240 F., the melting point of sulfur, and the flow rate is adjusted so that its downhole exit temperature through perforations 52 is 320 F. or less. The hot water flows out in a zone 60 between the steam zone 62 and the molten sulfur zone 48 and provides a blanket which insulates the molten sulfur pool 48 from the high-temperature steam zone.

A high-temperature steam zone moving out from the well is surrounded by a hot condensate zone with temperatures gradually declining from the condensation temperature (545 F. at 1,000 p.s.i.a. reservoir pressure) down to the reservoir ambient temperature. This decline in temperature is generally indicated by the dashed lines in FIG. 2. The advancing hot water front raises the temperature of the sulfur deposit above 240 F., and within the temperature range of 240-320 F. the sulfur melts and drains toward the pool of molten sulfur 48. The melted sulfur flows toward the well at low viscosity under the protection of the hot water blanket 60.

As mentioned above, one of the main causes of low thermal efficiency in the conventional Frasch process is the hot water leakage from the zone of molten sulfur capture. Since steam contains much more available heat per pound, it is estimated that the total water requirements per ton of produced sulfur for a high-temperature steam injection process can be reduced by at least a factor of five. It is desirable to regulate the steam injection rate, and to plug off reservoir voids and crevices by injection of mud, in order to achieve condensation of the steam within the effective zone of sulfur capture. By this practice a marked increase in thermal efficiency can be obtained over the conventional process due to the sharply reduced heat loss by hot water leakage. Thus, by the use of our system, we save valuable resources in the form of reduced volumes of water and fuel gas used for the heating process. Our process also is less costly and therefore permits the mining of sulfur from deposits which under the old system would be marginally economical.

While the above embodiments of the invention have been described with considerable detail, it is to be understood that various modifications of the device can be made without departing from the scope or spirit ofthe invention.

We claim:

1. A method of recovering sulfur from an underground deposit which comprises:

injecting steam at a temperature above about 325 F. into a first part of the deposit;

injecting hot water at not over about 320 F. into the deposit just below the point of injection of steam to form a hot water zone;

producing molten sulfur from below the hot water zone.

2. A method as defined in claim 1 in which said hot water is injected down a channel surrounding the channel through which the molten sulfur is produced.

3. A system for producing sulfur from an underground deposit which comprises:

a first casing set in a well drilled into said sulfur deposit, said casing terminating in the lower part of said deposit, there being steam injection perforations in the lower end of said casing;

a water injection tubing set inside said casing and the lower end portion extending below the said steam injection perforations, there being hot water injection perforations in the lower end portion of said hot water injection tubing to establish fluid communication between the hot water injection tubing and the said underground deposit of sulfur;

a sulfur-producing string suspended within said hot water injection tubing and extending below said hot water injec tion perforations;

a steam injection string of tubing inside said casing and exterior said hot water injection tubing, the lower end of said steam injection tubing terminating adjacent said steam injection perforations;

means to seal the lower interior end of said casing exterior of said hot water injection tubing above said hot water perforations;

means to close the interior of said casing exterior of said hot water injection tubing and said steam injection string just above said steam injection perforations;

means to close the lower end of said hot water injection tubing exterior of said sulfur-producing stringjust below said hot water perforations.

4. An apparatus or system as defined in claim 3 including means insulating said steam injection tubing string.

5. A system as defined in claim 3 including means to inject steam into said steam injection tubing string and means to inject hot water in the range of 2 50-300 F. into said hot water injection tubing.

6. A method as defined in claim 1 in which the steam is injected in the temperature range of from about 600 to 800 F.

7. A method as defined in claim 1 in which said steam is injected at a pressure over about 75 psi. 

2. A method as defined in claim 1 in which said hot water is injected down a channel surrounding the channel through which the molten sulfur is produced.
 3. A system for producing sulfur from an underground deposit which comprises: a first casing set in a well drilled into said sulfur deposit, said casing terminating in the lower part of said deposit, there being steam injection perforations in the lower end of said casing; a water injection tubing set inside said casing and the lower end portion extending below thE said steam injection perforations, there being hot water injection perforations in the lower end portion of said hot water injection tubing to establish fluid communication between the hot water injection tubing and the said underground deposit of sulfur; a sulfur-producing string suspended within said hot water injection tubing and extending below said hot water injection perforations; a steam injection string of tubing inside said casing and exterior said hot water injection tubing, the lower end of said steam injection tubing terminating adjacent said steam injection perforations; means to seal the lower interior end of said casing exterior of said hot water injection tubing above said hot water perforations; means to close the interior of said casing exterior of said hot water injection tubing and said steam injection string just above said steam injection perforations; means to close the lower end of said hot water injection tubing exterior of said sulfur-producing string just below said hot water perforations.
 4. An apparatus or system as defined in claim 3 including means insulating said steam injection tubing string.
 5. A system as defined in claim 3 including means to inject steam into said steam injection tubing string and means to inject hot water in the range of 250*-300* F. into said hot water injection tubing.
 6. A method as defined in claim 1 in which the steam is injected in the temperature range of from about 600* to 800* F.
 7. A method as defined in claim 1 in which said steam is injected at a pressure over about 75 p.s.i. 